U-Series

The U-Series Geochronometers

Unlike other dating tools described at earth-time.org, U-series encompasses a family of radioactive elements with different chemical characteristics that are related to each other through a chain of successive radioactive decays. The differing chemistries and half-lives of these nuclides (with timescales ranging from seconds to billions of years) make them exceptionally useful chronometers for variety of natural processes and materials Perhaps the most important and commonly used isotopes are 238U, 234U, 230Th and 226Ra, the first three of which are commonly used to date the formation of carbonate minerals and skeletal materials (e.g., corals and cave deposits) and the full suite of which are used to date volcanic materials, such as lavas and the crystals they contain.

Because these isotopes are related to each other by a radioactive decay chain, a material left chemically untouched for a long period of time (a “closed system”) exhibits a special condition called secular equilibrium, wherein the activity of each isotope in the chain is the same (activity is defined as the numbers of decays per unit time, which is equal to the number of atoms of that element times its decay constant). Materials that can be dated using U-series techniques are those that either (A) form by a process that causes disequilibrium, which results when the isotopes in the decay chain become separated (“fractionated”) in some way, or that (B) form by a process that records an existing disequilibrium in the material they form from. For instance, when crystals form in a magma, Th, U and Ra in the magma enter the different materials in different proportions, producing radioactive disequilibrium. Corals forming from seawater record the steady state 238U-234U-230Th disequilibrium of the water, which is recorded in the coral when it forms. Regardless of how the disequilibrium formed, over time the U-series isotopes will tend to return to a state of secular equilibrium so long as the material remains in a state that does not allow chemical exchanges in or out of it, remaining a “closed system”.

The time frame of this process, which is exploited to determine a U-series disequilibrium date, is governed by the shorter of the two half-lifes in any U-series isotope “parent-daughter” pair. The largest radioactive disequilibria are always found in the youngest materials. Over time, this signature goes away, eventually relaxing to a condition wherein the disequilibria are no longer detectable. How long this takes depends on the precision and accuracy of our measurements and the size of the original disequilibria (bigger disequilibria last longer). In practice, we can usually detect U-series disequilibria for 5 to 7 half-lifes. The half-lifes of 234U, 230Th and 226Ra are roughly 250, 75 and 1.6 thousand years, so that these isotopes are useful for looking at events that happened in the past thousand to million years. This is a very important time period of Earth history (the Pleistocene and Holocene) and a time period that very few other geochronometers can address.

Applications of U-series geochronology

The U-series chronometers can be used to date a wide variety of igneous, marine, terrestrial, and skeletal materials. A detailed discussion of every application is beyond the scope of this introduction, so we focus here on just the most common ones.

238U-234U-230Th dating of biogenic and abiogenic carbonate materials is a widely used application in paleoclimatology and paleooceanography. For instance, U-series dating of coral skeletons that grew in a specific environment and depth range relative to sea level can be used to reconstruct the history of sea level changes. This chronometer provides for internal checks on the closed-system requirement using the 234U/238U ratio because the value of 234U/238U is thought to change very little over the Pleistocene and modern eras. Fossil corals can exhibit some open-system behavior as they age, depending in part on the conditions they are in, such as diagenesis of coralline aragonite to calcite by meteoric water. Various methods have been proposed to mathematically correct or adjust for non-closed system behavior in corals to deduce an age. Although these methods are not universally accepted as robust, they do provide indications about relative ages and likely age assignments to important paleoclimate horizons, such as glacial and interglacial epochs. In a similar fashion to corals, calcium carbonate cave deposits formed slowly over time by precipitation from ground water can be dated to reconstruct sea level changes (e.g., when a cave became flooded with seawater and so deposition stopped) and, in conjunction with measurements of oxygen isotope variations in the layers of a deposit, can be used to develop very accurate chronologies of changes in climate (primarily temperature and groundwater composition). Unlike coralline aragonite, dense calcite cave formations are not as susceptible to diagenesis and have greater potential for preservation through time.
238U-230Th-226Ra dating of igneous materials takes several different forms and usually require a chemical normalization using a stable or long-lived isotope of the daughter isotope, much like is in used other radiometric systems like Rb-Sr and U-Pb. Here we use 232Th as a normalizing isotope for 238U and 230Th in Th-U dating and, because there is no longer-lived Ra isotope, we use chemically analogous Ba as a normalizer of 230Th and 226Ra in Ra-Th dating. The applications include the traditional mineral “isochron” approach (wherein minerals that grew initially from a magma with the same daughter isotope ratios (e.g., 230Th/232Th) and different U-Th ratios evolve in time to have different 230Th/232Th and minerals that formed together lie on a line, called an isochron in 230Th/232Th vs 238U/232Th space. This method is often called an “internal isochron” because the age of one rock is determined from the variations between minerals within it. While such ages are often used to infer when an eruption occurred, the event that is actually being dated is not the eruption per se, but the formation of minerals in a cooling magma, which happened sometime earlier. A modification of the internal isochron approach uses Ra-Th dating of minerals in historical eruptions to deduce the timescale over which the minerals themselves grew (by comparing their ages to the known eruption age). Another volcanic rock dating method using these isotopes looks at variations in daughter-parent isotope ratios between the whole-rock compositions of volcanic units of different ages at one volcano, and through a series of assumptions deduces the relative time between eruptions. This method is called an “external isochron” approach and generally yields somewhat approximate ages. This method is particularly useful for dating submarine lavas, where traditional radiocarbon methods (dating of eruption–related charcoal) is not possible. In addition, the data collected in this external isochron approach can also be used along with a melting model to determine that rates at which magmas form by melting in Earth’s mantle, allowing one to look at differences in melting rate, magma supply, and eruption volume at individual volcanoes.

In addition, there are important volcanic applications of shorter lived U-series isotopes 210Pb (22 year half-life) and 210Po (138 day half-life), expressed as the 226Ra-210Pb and 210Pb-210Po systems that are very useful for determining recent eruption chronologies and rapid mineral formation rates, based for instance on volatility differences between Po and Pb, and chemical differences between Ra and Pb (as well as the volatility of very short-lived isotopes between 226Ra and 210Pb (such as 222Rn). The 210Pb-210Po method was used for instance, to produce the very first eruption ages of suspected recent submarine eruptions on mid-ocean ridges, providing the final evidence for new crust generation there, as predicted by plate tectonic theory a half century before.

Contact details of some of the laboratories offering U-series geochronology in the US and elsewhere are listed below. This list includes the PIs of the U-series EARTHTIME initiative and a related expert advisory panel.

CIRDLES

CIRDLES is creating cyber infrastructure for geochronology in an interdisciplinary collaboration among our software engineers and geochemists from two NSF-supported initiatives, EARTHTIME and EarthChem.